as far as I know, the pleco should be fine with the color quartz. its the manufactured silica that might be a problem.
I switched from gravel to sand in an established tank. Here's how I did it (or should have, I made a mistake or two).
1. remove the fish
2. remove almost all the water.
3. remove all the gravel. I left a little bit, about a quart, I figured it would help seed bacterial colonies in the sand. its also kind of cool when my cichlids dig them up and spit them out on top of larger rocks.
4. Wash the new sand. that is where I messed up. I didn't wash it, it took over 30 minutes for the dust to settle in my room and 4 hours for the dust to settle in my tank. its probably best to do this first and have it ready when your tank is, you don't want your fish in temporary holding any longer than necessary. to wash it, just fill a bucket halfway and run a hose in it while stirring it with your hand until the water running over the top is clean.
5. Add the water. do a water change if it is way too cloudy.
6. Add the fish.
7. Enjoy and keep clean.
Edit: I used pool filter sand, but the procedure should be the same.